<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    ul{
      list-style: none;
      margin: 0;
      padding: 0;
      font-size: 0;
    }
    li{
      width: 200px;
      height: 50px;
      font-size: 16px;
      display: inline-block;/*行内样式不能设置宽高，所以设置行内块*/
    }
    li:nth-child(1){
      background-color: red;
    }
    li:nth-child(2){
      background: yellow;
    }
    li:nth-child(3){
      background-color: green;
    }
  </style>
</head>
<body>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
  </ul>
</body>
</html>